Coated fibrous copper microparticles, and electrically conductive coating agent and electrically conductive film each containing said coated fibrous copper microparticles

ABSTRACT

The present invention provides coated fibrous copper microparticles, wherein the coated fibrous copper microparticles are fibrous copper microparticles each at least partially coated with a metal other than copper, and the length and the aspect ratio of the fibrous copper microparticles are 1 μm or more and 10 or more, respectively.

TECHNICAL FIELD

The present invention relates to coated fibrous copper microparticles, and an electrically conductive coating agent and an electrically conductive film each containing the coated fibrous copper microparticles.

BACKGROUND ART

Spherical copper microparticles are a material excellent in electrical conductivity and inexpensive, and hence are widely used as raw materials for electrically conductive coating agents and the like. Such electrically conductive coating agents are widely used in materials for forming circuits on printed wiring boards or the like by using various printing methods, and various electrical contact members.

For the purpose of further improving the electrical conductivity and the stability of such copper microparticles, for example, coating the surface of the copper microparticles with a metal other than copper, typified by silver, has been proposed in many different ways. Such techniques have been disclosed in, for example, Japanese Patent Publication No. 57-59283, Japanese Patent Publication No. 2-46641 or Japanese Patent No. 4223754. These coated spherical copper microparticles are by no means inferior, in the electrical conductivity, to the metal microparticles composed only of an expensive noble metal such as silver, can reduce the production cost, and hence are valuable.

Recently, applications (for example, applications to touch panels, flat panel displays and the like) requiring electrically conductive materials having transparency (transparent electrically conductive materials) typified by transparent electrically conductive films have drastically expanded. Accordingly, the use of spherical copper microparticles as an electrically conductive material in the transparent electrically conductive materials for the electrically conductive coating agent and the electrically conductive layer has been investigated. For the electrically conductive coating agents, the capability of forming an electrically conductive layer is required.

However, in the case where such an electrically conductive coating agent as described above, containing conventional spherical copper microparticles coated with silver or the like is applied as the coating material for forming the electrically conductive layer of the transparent electrically conductive film utilized, for example, for a touch panel, when the content of the spherical copper microparticles coated with silver is increased in order to ensure sufficient electrical conductivity, the transparency in the electrically conductive layer is disadvantageously degraded. On the contrary, when the content of the copper microparticles coated with silver is reduced in order to ensure the transparency, the electrical conductivity in the electrically conductive layer is degraded. In other words, for the heretofore known transparent electrically conductive materials containing spherical copper microparticles coated with a noble metal such as silver, it is difficult to satisfy both of the electrical conductivity and the transparency required for the electrically conductive layer of the transparent electrically conductive film.

As described above, even if a transparent electrically conductive material is made to contain, as an electrically conductive material, spherical copper microparticles coated with a noble metal such as silver, it is difficult to satisfy both of the electrical conductivity and the transparency in the transparent electrically conductive material. On the other hand, microparticles composed only of a noble metal such as silver are expensive and sometimes disadvantageous in cost. Moreover, in order to obtain such microparticles as described above, a step of separating spherical microparticles is required, and hence the production of such metal microparticles disadvantageously requires huge time and labor.

SUMMARY OF INVENTION Technical Problem

Accordingly, an object of the present invention is to provide coated fibrous copper microparticles which solve the foregoing problems, and are excellent in both of the electrical conductivity and the transparency when contained in a transparent electrically conductive material.

Solution to Problem

Thus, the present inventors made a diligent study in order to solve the foregoing problems, and consequently have perfected the present invention by discovering for the first time that when the coated fibrous copper microparticles obtained by coating fibrous copper microparticles formed from copper, which is a metal drastically inexpensive as compared to silver, with a metal other than copper such as silver are contained in a transparent electrically conductive material, the transparent electrically conductive material is able to be an electrically conductive material excellent both in the electrical conductivity and the transparency.

Specifically, the gist of the present invention resides in the following.

(1) Coated fibrous copper microparticles, wherein the coated fibrous copper microparticles are fibrous copper microparticles each at least partially coated with a metal other than copper, and the length and the aspect ratio of the fibrous copper microparticles are 1 μm or more and 10 or more, respectively.

(2) The coated fibrous copper microparticles of (1), wherein the minor axis of the fibrous copper microparticles is 1 μm or less, and the proportion, in the fibrous copper microparticles, of copper particles having a minor axis of 0.3 μm or more and an aspect ratio of 1.5 or less is 0.1 or less per one fibrous copper microparticle.

(3) An electrically conductive coating agent including the coated fibrous copper microparticles of (1) or (2).

(4) An electrically conductive layer including the coated fibrous copper microparticles of (1) or (2).

(5) An electrically conductive film including a substrate, and the electrically conductive layer of (4), formed on the substrate.

Advantageous Effects of Invention

The coated fibrous copper microparticles of the present invention have specific shapes and constitutions such that the coated fibrous copper microparticles are fibrous copper microparticles the surface of each of which is at least partially coated with a metal other than copper, and the length and the aspect ratio of the fibrous copper microparticles are 1 μm or more and 10 or more, respectively. Accordingly, by using such coated fibrous copper microparticles, an electrically conductive coating agent, an electrically conductive layer and an electrically conductive film having both excellent electrical conductivity and excellent transparency can be obtained.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a view of the coated fibrous copper microparticles of the present invention, observed with a digital microscope.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the present invention is described in detail.

The coated fibrous copper microparticles of the present invention are coated fibrous copper microparticles obtained by at least partially coating the surface of each of fibrous copper microparticles, and the length and the aspect ratio of the fibrous copper microparticles are 1 μm or more and 10 or more, respectively.

As described above, the coated fibrous copper microparticles of the present invention are the fibrous copper microparticles the surface of each of which is coated with a metal other than copper, wherein as shown in FIG. 1, even after the coating with a metal, the fibrous shapes thereof are maintained. The fibrous copper microparticles coated with a metal other than copper are more excellent in the stability in a solvent or in the atmosphere as compared to the uncoated fibrous copper microparticles. Examples of the metal other than copper, for coating the fibrous copper microparticles include noble metal elements (such as gold, platinum, silver, palladium, rhodium, iridium, ruthenium and osmium) and base metal elements (such as iron, cobalt and tin). These may be used each alone or in combinations of two or more thereof. Among these, it is preferable to use at least silver from the viewpoint of the electrical conductivity and the stability.

The method for coating fibrous copper microparticles with a metal other than copper such as silver is not particularly limited, but as such a method, an electroless plating method is preferably used. In order to coat the surface of fibrous copper microparticles with a metal other than copper by adopting the electroless plating method, for example, in the case where coating with silver is performed, for example, the following methods can be used: a method in which by using silver nitrate, or a solution containing a silver complex of ammonium carbonate or ethylenediaminetetraacetate, silver is made to substitution deposit on the surface of the fibrous copper microparticles; or a method in which the fibrous copper microparticles are dispersed in a chelating agent solution, a silver nitrate solution is added to the resulting dispersion, subsequently a reducing agent is added to the dispersion, and thus the silver coat is deposited on the surface of the fibrous copper microparticles.

In order to coat fibrous copper microparticles with gold as a metal other than copper, a technique may be adopted in which, for example, as a gold source, chloroauric acid or gold potassium cyanide is used, and thus a gold coat is deposited on the surface of the fibrous copper microparticles. In order to coat fibrous copper microparticles with nickel, a technique may be adopted in which, for example, as a nickel source, nickel chloride or nickel acetate is used, and thus a nickel coat is deposited on the surface of the fibrous copper microparticles.

The fibrous copper microparticles metal not coated with a metal (namely, uncoated fibrous copper microparticles) are described.

The length of the fibrous copper microparticles is required to be 1 μm or more, and is preferably 5 μm or more and more preferably 10 μm or more. When the length of the fibrous copper microparticles is less than 1 μm, in a transparent electrically conductive material containing the coated fibrous copper microparticles of the present invention, it is difficult to make the satisfactory electrical conductivity and the satisfactory transparency compatible with each other. On the other hand, from the viewpoint of the handling of the coating agent at the time of formation of an electrically conductive layer or the electrically conductive film containing the coated fibrous copper microparticles of the present invention, it is sometimes preferable for the length of the fibrous microparticles not to exceed 500 μm.

The minor axis of the fibrous copper microparticles is preferably 1 μm or less, more preferably 0.5 μm or less, furthermore preferably 0.2 μm or less and particularly preferably 0.1 μm or less. When the minor axis of the fibrous copper microparticles exceeds 1 μm, the transparency of the transparent electrically conductive material containing the coated fibrous copper microparticles of the present invention is sometimes poor.

The aspect ratio (length of fibrous body/minor axis of fibrous body) of the fibrous copper microparticles is required to be 10 or more, is preferably 100 or more and more preferably 300 or more. When the aspect ratio of the fibrous copper microparticles is less than 10 (namely, closer to sphere), it is difficult to made the transparency and the electrical conductivity compatible with each other in the transparent electrically conductive material containing the coated fibrous copper microparticles of the present invention.

In the coated fibrous copper microparticles of the present invention, the entire surface of the coated fibrous copper microparticles is preferably coated with a metal other than copper; however, the surface of the coated fibrous copper microparticles may also be partially free from coating so as to be partially exposed. The content of the coating metal other than copper in the coated fibrous copper microparticles, in relation to the mass of the whole of the coated fibrous copper microparticles, is preferably 1 to 50% by mass, more preferably 10 to 50% by mass and furthermore preferably 15 to 30% by mass. When the content of the coating metal other than copper is less than 1% by mass, the improvement of the electrical conductivity as the effect achieved by metal coating is sometimes insufficient. On the other hand, when the content of the coating metal other than copper exceeds 50% by mass, there is a possibility such that the cost of materials due to the coating with a metal other than copper is increased, or the minor axis of the coated fibrous copper microparticles is increased. The coating amount of a metal can be determined, for example, by obtaining a measurement solution through dissolving in a strong acid the coated fibrous copper microparticles of the present invention and by performing a measurement of the resulting solution on the basis of ICP (induced coupled plasma).

The methods for determining the minor axis and the length (major axis) of the fibrous copper microparticles and the below-described copper particles and the method for deriving the number of the copper particles per one fibrous copper particle are as follows.

Specifically, for example, a transmission electron microscope (TEM) or a scanning electron microscope (SEM) is used, and the aggregates of the fibrous copper microparticles are observed. For the observation of the fibrous copper microparticles, for example, digital microscope (“VHX-1000, and VHX-D500/510,” manufactured by Keyence Corp.) can be used.

From the aggregates, 100 fibrous copper microparticles were selected. The selected fibrous copper microparticles and the copper particles attaching to or contacting with the fibrous copper microparticles are each subjected to the measurement of the minor axis and the length, and the resulting average values can be taken as the minor axis and the length. By dividing the thus derived length by the thus derived minor axis, the aspect ratio of the fibrous copper microparticles and the aspect ratio of the copper particles can be derived. Moreover, by counting the number of the copper particles and by dividing the number of the copper particles by the number (100) of the fibrous copper microparticles, the number of the copper particles per one fibrous copper microparticle can be derived.

In this connection, when the fibrous copper microparticles of the present invention are observed, in the case where the fibrous copper microparticles overlap each other and crowd each other, it is sometimes impossible to accurately evaluate the shapes of the fibrous copper microparticles and the shapes of the copper particles. Accordingly, in such a case, by using, for example, an ultrasonic disperser, the crowding fibrous copper microparticle can be disentangled to such an extent that the neighboring fibrous copper microparticles are not in close contact with each other.

In the uncoated fibrous copper microparticles, the proportion of the copper particles having a minor axis of 0.3 μm or more and an aspect ratio of 1.5 or less is preferably 0.1 or less, more preferably 0.08 or less, furthermore preferably 0.05 or less and most preferably null per one fibrous copper microparticle. When the copper particles are present in a proportion exceeding 0.1 per one fibrous copper microparticle, the transparency of the transparent electrically conductive material containing the coated fibrous copper microparticles of the present invention is sometimes poor.

The minor axis and the aspect ratio (length of copper particles/minor axis of copper particles) of the copper particles, affecting the transparency is 0.3 μm or more and 1.5 or less, respectively.

For the purpose of producing such uncoated fibrous copper microparticles as described above, for example, the following method is used. Specifically, a method is used which precipitates fibrous copper microparticles from an aqueous solution containing copper ion, an alkaline compound, a nitrogen-containing compound capable of forming a stable complex with copper ion and a reducing compound. In this case, it is preferable to use as the reducing compound a compound not reacting with the dissolved oxygen in the alkaline aqueous solution.

If a compound reacting with the dissolved oxygen in the alkaline aqueous solution is used as the reducing compound, in the obtained fibrous copper microparticles, the proportion of the copper particles sometimes exceeds 0.1 per one fibrous copper microparticle; in other words, sometimes the fibrous copper microparticles having many copper particles are obtained.

Here, “the reducing compound not reacting with dissolved oxygen” is defined by the following index.

First, to 500 g of pure water, a few drops of a 10% aqueous solution of sodium hydroxide were added to prepare an aqueous solution (water temperature: 25° C.) adjusted to pH 10.4. The dissolved oxygen concentration of this alkaline aqueous solution is taken as “the dissolved oxygen concentration 1.” Specifically, the dissolved oxygen concentration 1 is 8.3 mg/L. For the measurement of the dissolved oxygen concentration, for example, a dissolved oxygen meter “DO-5509” (manufactured by Lutron Electronic Enterprise Co., Ltd.) is used.

Subsequently, 100 mL of the alkaline aqueous solution is placed in an open cylindrical vessel of 7.0 cm in diameter, then a reducing compound is added to the alkaline aqueous solution so as to have a concentration of 0.50 mol/L, and is dissolved by stirring the aqueous solution with a magnetic stirrer to a degree such that the aqueous solution is not whirled. While after the dissolution, successively the aqueous solution is being continuously stirred, the dissolved oxygen concentration in the aqueous solution is measured at the elapsed times after the addition of the reducing compound of 0.5 minute, 5 minutes, 10 minutes, 15 minutes and 30 minutes. And, the dissolved oxygen concentration at the elapsed time after the addition of the reducing compound of 10 minutes is taken as “the dissolved oxygen concentration 2.”

Then, the numerical value A is determined by the following formula (1).

A=(dissolved oxygen concentration 2)/(dissolved oxygen concentration 1)  (1)

In the present invention, the reducing compound giving the numerical value A obtained from formula (1) of 0.5 or more is defined as “the reducing compound not to react with the dissolved oxygen.” And, the reducing compound giving the numerical value A of less than 0.5 is defined as “the reducing compound to react with the dissolved oxygen.”

Examples of the reducing compound not to react with the dissolved oxygen include ascorbic acid, erythorbic acid, glucose and hydroxylammonium salt. The numerical values A of these reducing compound not to react with the dissolved oxygen are all 0.5 or more. Among these, it is preferable to use one or more selected from ascorbic acid, erythorbic acid and glucose, and it is most preferable to use ascorbic acid.

In the prior art, in the production of fibrous copper microparticles, in general, the copper microparticles are precipitated by using hydrazine as the reducing compound contained in the reaction solution. However, when “the reducing compound to react with the dissolved oxygen” such as hydrazine is used, only fibrous copper microparticles increased in the proportion of copper particles are sometimes obtained. Alternatively, the fibrous copper microparticles themselves cannot sometimes be precipitated.

It is to be noted that in hydrazine, a conventionally used reducing compound, the numerical value A obtained from the foregoing formula (1) is approximately 0.05.

In the present invention, it is preferable to maintain within a high range the dissolved oxygen concentration in the aqueous solution for precipitating the fibrous copper microparticles. More specifically, as the water contained in the aqueous solution, water having a dissolved oxygen concentration of 1 mg/L or more is preferably used, and water having a dissolved oxygen concentration of 3 mg/L or more is more preferably used. When water having a dissolved oxygen concentration of less than 1 mg/L is used, the proportion of the copper particles per one fibrous copper microparticle exceeds 0.1, and accordingly, when such fibrous copper microparticles are contained in a transparent electrically conductive material, only fibrous copper microparticles poor in transparency can sometimes be obtained.

Such a reducing compound as described above is used, in relation to the copper ion in the aqueous solution, preferably in a proportion of 0.5 to 5.0 molar equivalent and more preferably in a proportion of 0.75 to 3.0 molar equivalent. When the reducing compound is used in a proportion of less than 0.5 molar equivalent, the formation efficiency of the fibrous copper microparticles is sometimes degraded. On the other hand, also when the reducing compound is used in a proportion exceeding 5.0 molar equivalent, the formation effect of the fibrous copper microparticles is saturated to be unfavorable from the viewpoint of, for example, the cost.

The copper ion can be produced by dissolving a water-soluble copper salt in water. Examples of the water-soluble copper salt include copper sulfate, copper nitrate, copper chloride and copper acetate. Among these, copper sulfate or copper nitrate can be preferably used from the viewpoint of the easiness in forming the fibrous copper microparticles of the present invention.

The alkaline compound is not particularly limited; for example, sodium hydroxide and potassium hydroxide can be used.

The concentration of the alkaline compound in the aqueous solution is set preferably at 15 to 50% by mass, more preferably at 30 to 50% by mass and furthermore preferably at 35 to 45% by mass. When the concentration of the alkaline compound is less than 15% by mass, it is sometimes difficult to form the fibrous copper microparticles of the present invention. On the other hand, when the concentration exceeds 50% by mass, the handling of the aqueous solution is sometimes difficult.

The concentration of copper ion in the aqueous solution is specified by the molar ratio between the hydroxide ion of the alkaline compound and the copper ion. Specifically, (hydroxide ion of alkaline compound)/(copper ion) is preferably set so as to fall within a range from 3000/1 to 6000/1 and more preferably 3000/1 to 5000/1 in terms of molar ratio. When the molar ratio is less than 3000/1, the formation of the copper particles cannot be suppressed, and consequently the proportion of the copper particles exceeds 0.1 per one fibrous copper microparticle, or alternatively, the shape of the copper microparticles is sometimes not fibrous but spherical. On the other hand, when the molar ratio exceeds 6000/1, the formation efficiency of the fibrous copper microparticles is sometimes poor.

Examples of the nitrogen-containing compound that forms a stable complex with the divalent copper ion in an aqueous solution include ammonia, ethylenediamine and triethylenetetramine. Among these, ethylenediamine can be preferably used from the viewpoint of the easiness in forming the fibrous copper microparticles.

The nitrogen-containing compound is preferably used in a proportion of 1 mole or more in relation to 1 mole of copper ion from the viewpoint of the formation efficiency of the fibrous copper microparticles.

Subsequently, the aqueous solution containing such components as described above is heated with an appropriate heat source, and then, by continuing the heating of the aqueous solution or by decreasing the temperature of the aqueous solution, the precipitation of the intended fibrous copper microparticles can be made to occur. In particular, the latter method, namely, the method decreasing the solution temperature after heating is more preferable.

The temperature for heating the aqueous solution is not particularly limited, but is preferably 50 to 100° C. from the viewpoint of the balance between the precipitation efficiency and the cost.

The precipitated fibrous copper microparticles are subjected to solid-liquid separation by a method such as filtration, centrifugation or pressure floatation, and thus can be collected. Moreover, if necessary, the collected fibrous copper microparticles may be washed or dried. When the fibrous copper microparticles are taken out, it is preferable to perform the taking-out operation in an inert gas atmosphere (for example, nitrogen atmosphere) because the surface of the fibrous copper microparticles tends to be oxidized.

When the taken-out fibrous copper microparticles are stored, the storage is preferably performed in an inert gas atmosphere such as nitrogen gas atmosphere, or alternatively preferably performed by redispersing the taken-out fibrous copper microparticles in a solution such as a solution prepared by dissolving a small amount of a reducing compound, or a solution prepared by dissolving a small amount of an organic substance having a function of preventing the oxidation of copper.

Alternatively, the fibrous copper microparticles precipitated by such a method as described above are collected by performing solid-liquid separation, then washed with water or a solution prepared by dissolving a small amount of a reducing compound such as ascorbic acid, and may be subjected to the step of coating with a metal other than copper immediately after the washing without performing the storage in a state of the fibrous copper microparticles so as to yield the coated fibrous copper microparticles of the present invention. This method is more preferable from the viewpoint of suppressing the surface oxidation of the fibrous copper microparticles.

The electrically conductive coating agent of the present invention can be prepared by mixing and dispersing, in a binder component and a solvent, the coated fibrous copper microparticles of the present invention obtained by coating the fibrous copper microparticles having a specific shape with a metal other than copper.

The binder component is not particularly limited, and examples of the usable binder components include: acrylic resins (such as silicone-modified acrylic resin, fluorine-modified acrylic resin, urethane-modified acrylic resin and epoxy-modified acrylic resin); polyester-based resin, polyurethane-based resin, olefin-based resin, amide resin, imide resin, epoxy resin, silicone resin and vinyl acetate-based resin; natural polymers such as starch, gelatin and agar; semisynthetic polymers such as cellulose derivatives such as carboxymethyl cellulose, hydroxy ethyl cellulose, methyl cellulose, hydroxy ethyl methyl cellulose, hydroxy propyl methyl cellulose; synthetic polymers such as water-soluble polymers such as polyvinyl alcohol, polyacrylic acid-based polymer, polyacrylamide, polyethylene oxide and polyvinylpyrrolidone.

The solvent is not particularly limited, and examples of the solvent include: water, and organic solvent such as alcohols, glycols, cellosolves, ketones, esters, ethers, amides and hydrocarbons. These can be used each alone or in combinations of two or more thereof. Among these, it is preferable to use a solvent mainly composed of water or an alcohol.

The mixing ratio between the coated fibrous copper microparticles and the binder in the electrically conductive coating agent of the present invention is, in terms of the volume ratio (A/B) between the volume (A) of the coated fibrous copper microparticles and the volume (B) of the binder, preferably 1/100 to 5/1 and more preferably 1/20 to 1/1. When the amount of the coated fibrous copper microparticles is small to such an extent that the volume ratio of the coated fibrous copper microparticles to the binder is less than 1/100, the electrical conductivity is sometimes low in the obtained electrically conductive coating agent or the electrically conductive layer obtained from the coating agent. On the other hand, when the amount of the binder is small to such an extent that the volume ratio exceeds 5/1, the electrically conductive layer prepared from the coating agent is sometimes poor in surface smoothness or in transparency, or the adhesiveness between the substrate and the electrically conductive layer, which is formed on the substrate by coating the substrate with the electrically conductive coating agent, is sometimes degraded.

The solid content (the total content of the coated fibrous copper microparticles and binder of the present invention, and the solid content of the other additive(s) added according to need) is preferably 1 to 99% by mass and more preferably 1 to 50% by mass from the viewpoint of being excellent in the balance, for example, between the electrical conductivity and the handleability.

The viscosity of the electrically conductive coating agent of the present invention at 20° C. is preferably 0.5 to 100 mPa·s and more preferably 1 to 50 mPa·s from the viewpoint of being excellent, for example, in the handleability and the easiness to coat a substrate.

In the electrically conductive coating agent of the present invention, a cross-linking agent such as an aldehyde-based, epoxy-based, melamine-based or isocyanate-based cross-linking agent may also be used, if necessary, within a range not impairing the advantageous effects of the present invention.

By forming a layer with the electrically conductive coating agent of the present invention, the electrically conductive layer of the present invention can be obtained. Moreover, by forming the electrically conductive layer on a substrate, the electrically conductive film of the present invention can be obtained. The electrically conductive layer and the electrically conductive film of the present invention are excellent both in transparency and in electrical conductivity.

Examples of the method for forming the electrically conductive layer include a method in which the surface of a substrate such as a plastic film is coated with the electrically conductive coating agent of the present invention, subsequently the electrically conductive coating agent on the substrate is dried, and then, if necessary, cured to form a layer, namely, a so-called wet coating formation method. As the coating method, for example, the following methods can be used: a roll coating method, a bar coating method, a dip coating method, a spin coating method, a casting method, a die coating method, a blade coating method, a gravure coating method, a curtain coating method, a spray coating method and a doctor coating method.

The thickness of the electrically conductive layer may be, for example, about 0.1 to 10 μm from the viewpoint of practicability.

In order to form an electrically conductive layer or an electrically conductive film containing the coated fibrous copper microparticles of the present invention, a method can also be used in which only the coated fibrous copper microparticles of the present invention are applied to the surface of a substrate such as a plastic film, and if necessary, a coating layer for protecting the applied coated fibrous copper microparticles is formed.

EXAMPLES

Hereinafter, the present invention is specifically described with reference to Examples. It is to be noted that the present invention is not limited by these Examples.

The evaluation methods and the measurement methods for the coated fibrous copper microparticles obtained in Examples and the uncoated fibrous copper microparticles or the fibrous silver microparticles used in Comparative Examples are as follows.

1. Evaluation of Reducing Compound not Reacting with Dissolved Oxygen

On the basis of the evaluation standard for the reaction between the reducing compound and the dissolved oxygen, based on the foregoing formula (1) [namely, A=(dissolved oxygen concentration 2)/(dissolved oxygen concentration 1)], the reactivity between the reducing compound used in each of Examples and Comparative Examples and the dissolved oxygen was evaluated.

The dissolved oxygen concentration 1 is the dissolved oxygen concentration in an alkaline aqueous solution, measured in the foregoing manner. The dissolved oxygen concentration 2 is the dissolved oxygen concentration in an aqueous solution after 10 minutes from the addition of the reducing compound, measured in the foregoing manner.

2. Dissolved Oxygen Concentration in Alkaline Aqueous Solution

The titled concentration was measured by using a dissolved oxygen meter “DO-5509” (manufactured by Lutron Electronic Enterprise Co., Ltd.).

3. Minor Axis and Length of Fibrous Copper Microparticles

The aggregates of the fibrous copper microparticles were prepared, and were lightly disentangled by using an ultrasonic disperser in order that the fibrous copper microparticles might not be in close contact with each other. Then, the fibrous copper microparticles were observed by using a digital microscope (“VHX-1000, VHX-D500/510,” manufactured by Keyence Corp.). From the aggregates, 100 of the fibrous copper microparticles were selected, the minor axis and the length of the fibrous copper microparticles and the copper particles were measured, and the average values of these measured values were taken as the minor axis and the length.

4. Aspect Ratios of Fibrous Copper Microparticles and Copper Particles

The aspect ratio was derived for the fibrous copper microparticles and the copper particles, by dividing the concerned length determined in the foregoing 3. by the concerned minor axis determined in the foregoing 3.

5. Number of Copper Particles Per One Fibrous Copper Microparticle

The aggregates of the fibrous copper microparticles were prepared, and were lightly disentangled by using an ultrasonic disperser in order that the fibrous copper microparticles might not be in close contact with each other. Then, the fibrous copper microparticles were observed by using a digital microscope (“VHX-1000, VHX-D500/510,” manufactured by Keyence Corp.). From the aggregates, 100 of the fibrous copper microparticles were selected, the number of the copper particles in the fibrous copper microparticles was counted, and the number of the copper particles per one fibrous copper microparticle was derived by dividing the number of the copper particles by the number (100) of the fibrous copper microparticles.

6. Coating Amount of Metal in Fibrous Copper Microparticles

The coated fibrous copper microparticles obtained in each of Examples were sampled in a glass beaker, dissolved in nitric acid and diluted, and the resulting solution was used as the measurement solution. For the measurement solution, a quantitative evaluation was performed with ICP (manufactured by Nippon Jarrell-Ash Co., Ltd.). From the content ratio between the respective quantitatively determined metals (namely, copper and the metal other than copper), the coating amount of the metal in the fibrous copper microparticles was derived. In Examples of the present description, silver was used as the metal other than copper, and hence the coating amount of a metal means the coating amount of silver.

7. Stability of Coated Fibrous Copper Microparticles and Stability of Uncoated Fibrous Copper Microparticles

The coated fibrous copper microparticles obtained in each of Examples and the uncoated fibrous copper microparticles used in each of the Comparative Examples were immersed in water for 7 days, and allowed to stand still at room temperature. Subsequently, by the X-ray diffraction method using “RINT-TTR III” manufactured by Rigaku Corp., the presence/absence of the peaks of copper and the substance(s) (for example, copper oxide) other than silver on the surface of the coated fibrous copper microparticles or the uncoated fibrous copper microparticles was identified to perform the detection of the foregoing substance(s). The evaluation of the stability was performed on the basis of the following standards.

G (Good): the substance(s) other than copper and silver were not detected.

P (Poor): the substance(s) other than copper and silver were detected.

8. Volume Resistivity and Resistance Value Change of Coated Fibrous Copper Microparticles (Unit: Ω·cm)

The coated fibrous copper microparticles obtained in each of Examples or the uncoated fibrous copper microparticles used in each of Comparative Examples were dispersed in an aqueous solution (10% by mass) of ascorbic acid, and then collected by pressure filtration (filter: PTFE membrane filter having a pore size of 1 μm, manufactured by Advantec Toyo Kaisha, Ltd.) with nitrogen, to prepare a sample in which the microparticles were laminated in a sheet-like shape on the filter. The obtained sample was dried with a dryer at 60° C. for 30 minutes under normal pressure and then subjected to a reduced-pressure drying treatment for 1 hour. By using a resistivity meter (Loresta AP, MCP-T400, manufactured by Dia Instruments Co., Ltd.), the volume resistivity of each set of the microparticles laminated in a sheet-like shape was measured.

Next, the sample was heat treated at 180° C. for 1 hour in air atmosphere in the dryer, then the volume resistivity of each set of the microparticles laminated in a sheet-like shape was measured in the same manner as described above, and thus the change of the resistance value due to the heat treatment was evaluated.

Production Example 1 of Uncoated Fibrous Copper Microparticles

In a 300-mL three-necked flask, 108.0 g of sodium hydroxide (manufactured by Nacalai Tesque, Inc.) as an alkaline compound was dissolved in 180.0 g of pure water (dissolved oxygen concentration at 27° C.: 8.7 mg/L). Next, to the resulting aqueous solution, an aqueous solution prepared by dissolving 0.15 g of copper nitrate trihydrate (manufactured by Nacalai Tesque, Inc.) as a copper salt for producing copper ion in 6.2 g of pure water, and 0.81 g of ethylene diamine (manufactured by Nacalai Tesque, Inc.) as a nitrogen-containing compound were added and stirred at 200 rpm to prepare a uniform blue aqueous solution. Here, the molar ratio between the hydroxide ion and the copper ion in the aqueous solution was set at 4500/1.

To the aqueous solution, 1.2 g of an aqueous solution (4.4% by mass) of ascorbic acid (manufactured by Nacalai Tesque, Inc., the numerical value A: 0.88) as a reducing compound was added, and the three-necked flask was immersed in a hot water bath set at 80° C. while the stirring of the solution at 200 rpm was being continued. The color of the solution gradually changed from blue to light color, and the solution was almost colorless and transparent after 30 minutes.

After a further elapsed time of 30 minutes, 4.8 g of the aqueous solution (4.4% by mass) of ascorbic acid as a reducing compound was added and the stirring of the solution was continued for about 1 minute. Then, the stirring was terminated, the three-necked flask was taken out from the hot water bath, and the precipitation of the fibrous copper microparticles in the cooling process was visually verified. It is to be noted that the inside of the three-necked flask was in a state of being filled with air during the reaction.

The precipitated fibrous copper microparticles were collected by pressure filtration (PTFE membrane filter having a pore size of 1 μm, manufactured by Advantec Toyo Kaisha, Ltd.) with nitrogen, washed once with an aqueous solution (10% by mass) of ascorbic acid and three times with pure water, and then dried in a dryer at 50° C. The fibrous copper microparticles thus obtained were referred to as “the uncoated fibrous copper microparticles 1.” The uncoated fibrous copper microparticles 1 were subjected to the foregoing evaluations 3., 4. and 5. The evaluation results thus obtained are shown in Table 1. The evaluation results are shown under the item headings of the shape of the fibrous copper microparticles in Examples 1 to 4 and Comparative Example 1.

TABLE 1 Shape of fibrous cooper Properties of coated fibrous copper microparticles microparticles Number of copper Volume resistivity particles (per Content of (Ω · cm) Minor one fibrous coating After axis Length Aspect copper Coating metal (% heat (μm) (μm) ratio microparticle) metal by mass) Stability Initial treatment Example 1 0.07 122 1740 0.05 Silver 48 G 7.7 × 10⁻⁵ 9.4 × 10⁻⁵ Example 2 0.07 122 1740 0.05 Silver 70 G 6.7 × 10⁻⁵ 6.6 × 10⁻⁵ Example 3 0.07 122 1740 0.05 Silver 24 G 8.0 × 10⁻⁵ 2.5 × 10⁻⁴ Example 4 0.07 122 1740 0.05 Silver 13 G 9.5 × 10⁻⁵ 7.9 × 10⁻⁴ Example 5 0.3 14 47 0.5 Silver 50 G — — Comparative 0.07 122 1740 0.05 Absent — P 3.5 × 10⁻⁴ 2.2 × 10²  Example 1 Comparative 0.3 14 47 0.5 Absent — P — — Example 2 * In the table, the number of the copper particles is the number of the copper particles having a minor axis of 0.3 μm or more and an aspect ratio of 1.5 or less, contained per one fibrous copper microparticle having an aspect ratio of 10 or more.

Production Example 2 of Uncoated Fibrous Copper Microparticles

In a 300-mL three-necked flask, 108.0 g of sodium hydroxide (manufactured by Nacalai Tesque, Inc.) as an alkaline compound was dissolved in 180.0 g of pure water (dissolved oxygen concentration at 27° C.: 8.7 mg/L). Next, to the resulting aqueous solution, an aqueous solution prepared by dissolving 0.22 g of copper nitrate trihydrate (manufactured by Nacalai Tesque, Inc.) as a copper salt for producing copper ion in 9.2 g of pure water, and 1.2 g of ethylene diamine (manufactured by Nacalai Tesque, Inc.) as a nitrogen-containing compound were added and stirred at 200 rpm to prepare a uniform blue aqueous solution. Here, the molar ratio between the hydroxide ion and the copper ion in the aqueous solution was set at 3000/1.

To the solution, 0.34 g of an aqueous solution (55% by mass) of hydrazine monohydrate (manufactured by Wako Pure Chemical Industries, Ltd., the numerical value A: 0.05) as a reducing compound was added, and the three-necked flask was immersed in a hot water bath set at 80° C. while the stirring of the solution at 200 rpm was being continued. Immediately, the color of the solution changed from blue and the solution turned colorless and transparent, and precipitates occurred. Subsequently, after 60 minutes, the three-necked flask was taken out from the hot water bath. It is to be noted that the inside of the three-necked flask was in a state of being filled with air during the reaction. The obtained precipitates were collected by the same method as in Production Example 1, and the fibrous copper microparticles thus obtained were referred to as “the uncoated fibrous copper microparticles 2.” The uncoated fibrous copper microparticles 2 were subjected to the foregoing evaluations 3., 4. and 5. The evaluation results thus obtained are shown in Table 1. The evaluation results are shown under the item headings of the shape of the fibrous copper microparticles in Example 5 and Comparative Example 2.

Example 1

In a plastic vessel with a stirrer chip placed therein, 0.01 g of “the uncoated fibrous copper microparticles 1” and 18 g of an aqueous solution (10% by mass) of ascorbic acid were added to prepare a suspension. While the suspension was being stirred at room temperature at 700 rpm, 2 g of a predip solution for substitution-type electroless silver plating (“SSP-700P” manufactured by Shikoku Chemicals Corp.) was added, and after the addition, the stirring was continued for 5 minutes. While the stirring at 700 rpm was still being continued, a solution prepared by mixing 0.5 g of a substitution-type electroless silver plating solution (“SSP-700M” manufactured by Shikoku Chemicals Corp.) and 19.5 g of ion-exchanged water was dropwise added to the suspension over 5 minutes, and then the color of the suspension changed from reddish brown to light brown.

The suspension was subjected to pressure filtration (filter: PTFE membrane filter having a pore size of 1 μm, manufactured by Advantec Toyo Kaisha, Ltd.) with nitrogen, and the filtered-off substance was washed by making ion-exchanged water pass through the filtered-off substance to prepare a sample in which the microparticles were laminated in a sheet-like shape on the filter. The sample was dried in a dryer at 60° C., and thus, the fibrous copper microparticles coated with silver were obtained in a state of being deposited on the filter. The obtained coated fibrous copper microparticles were subjected to the foregoing evaluations 6., 7. and 8. The evaluation results thus obtained are shown in Table 1.

Example 2

Fibrous copper microparticles coated with silver were obtained by the same method as in Example 1 except that in Example 1, in the solution prepared by mixing 0.5 g of the substitution-type electroless silver plating solution (“SSP-700M”) and 19.5 g of ion-exchanged water, the mixing amounts were altered to 1 g and 19 g, respectively. The obtained coated fibrous copper microparticles were subjected to the same evaluations as in Example 1. The evaluation results thus obtained are shown in Table 1.

Example 3

Fibrous copper microparticles coated with silver were obtained by the same method as in Example 1 except that in Example 1, in the solution prepared by mixing 0.5 g of the substitution-type electroless silver plating solution (“SSP-700M”) and 19.5 g of ion-exchanged water, the mixing amounts were altered to 0.2 g and 19.8 g, respectively. The obtained coated fibrous copper microparticles were subjected to the same evaluations as in Example 1. The evaluation results thus obtained are shown in Table 1.

Example 4

Fibrous copper microparticles coated with silver were obtained by the same method as in Example 1 except that in Example 1, in the solution prepared by mixing 0.5 g of the substitution-type electroless silver plating solution (“SSP-700M”) and 19.5 g of ion-exchanged water, the mixing amounts were altered to 0.1 g and 19.9 g, respectively. The obtained coated fibrous copper microparticles were subjected to the same evaluations as in Example 1. The evaluation results thus obtained are shown in Table 1.

Example 5

Fibrous copper microparticles coated with silver were obtained by the same method as in Example 1 except that “the uncoated fibrous copper microparticles 2” were used in place of “the uncoated fibrous copper microparticles 1.” The obtained coated fibrous copper microparticles were subjected to the same evaluations as in Example 1 except for the item of volume resistivity. The evaluation results thus obtained are shown in Table 1.

Comparative Example 1

“The uncoated fibrous copper microparticles 1” were subjected to the same evaluations as in Example 1, without performing the coating treatment with a metal. The evaluation results thus obtained are shown in Table 1.

Comparative Example 2

“The uncoated fibrous copper microparticles 2” were subjected to the same evaluations as in Example 1 except for the item of volume resistivity, without performing the coating treatment with a metal. The evaluation results thus obtained are shown in Table 1.

Comparative Example 3

The pressure filtration (PTFE membrane filter having a pore size of 1 μm, manufactured by Advantec Toyo Kaisha, Ltd.) with nitrogen was performed for 6 g of a silver microparticles dispersion (product number: 739448, a dispersion containing 0.5% by mass of silver microparticles dispersed in isopropanol, manufactured by Aldrich Corp.), wherein the microparticles as fibrous silver microparticles had a minor axis of 0.1 μm and a length of 30 μm; thus, a sample in which the fibrous silver microparticles were laminated in a sheet-like shape on the filter was prepared. The sample was dried with a dryer at 60° C. for 30 minutes under normal pressure and then subjected to a reduced-pressure drying treatment for 1 hour.

The sample after the reduced-pressure drying treatment was subjected to a volume resistivity measurement with a resistivity meter (Loresta AP, MCP-T400 manufactured by Dia Instruments Co., Ltd.). The initial volume resistivity value was found to be 5.7×10⁻⁵ (Ω·cm). Subsequently, the sample was subjected to a heat treatment at 180° C. for 1 hour, and volume resistivity value after the heat treatment was found to be 5.0×10⁻⁵ (Ω·cm).

The coated fibrous copper microparticles obtained in each of Examples 1 to 5 were simply obtained from fibrous copper microparticles having a length of 1 μm or more and an aspect ratio of 10 or more, and were excellent in stability.

In particular, the coated fibrous copper microparticles obtained in each of Examples 1 to 4 were found to have a minor axis of 1 μm or less and a very large aspect ratio, and were obtained by coating with silver fibrous copper microparticles having a small proportion of copper particles having a minor axis of 0.3 μm or more and an aspect ratio of 1.5 or less. Accordingly, as compared to the uncoated fibrous copper microparticles (Comparative Example 1), the volume resistivity values were low, and in other words, exhibited satisfactory electrical conductivity. The electrical conductivity was almost the same as that of the fibrous silver microparticles (Comparative Example 3), and was by no means inferior to the electrical conductivity of fibrous microparticles composed only of silver.

On the other hand, in Comparative Examples 1 and 2, evaluations were performed by using fibrous copper microparticles the surface of which was not coated with a metal other than copper. Such fibrous copper microparticles did not have satisfactory properties with respect to the stability.

Examples 1 to 4 for the first time show that it is possible to control the silver coating amount for the fibrous copper microparticles having a length of 1 μm or more and an aspect ratio of 10 or more. The change of the volume resistivity value due to the heat treatment of the silver coated fibrous copper microparticles in each of Examples 1 to 4 was almost not found similarly to the case of the fibrous silver microparticles shown in Comparative Example 3, and thus the foregoing volume resistivity was a satisfactory property. On the other hand, in Comparative Example 1, the fibrous copper microparticles the surface of which was not coated with a metal other than copper were adopted, and accordingly the volume resistivity after heat treatment was remarkably increased and the electrical conductivity was degraded.

INDUSTRIAL APPLICABILITY

By using the coated fibrous copper microparticles of the present invention, an electrically conductive coating agent, an electrically conductive layer and an electrically conductive film having both excellent electrical conductivity and excellent transparency can be obtained, and hence the coated fibrous copper microparticles of the present invention is extremely useful. 

1. Coated fibrous copper microparticles, wherein the coated fibrous copper microparticles are fibrous copper microparticles each at least partially coated with a metal other than copper, and a length and an aspect ratio of the fibrous copper microparticles are 1 μm or more and 10 or more, respectively.
 2. The coated fibrous copper microparticles according to claim 1, wherein a minor axis of the fibrous copper microparticles is 1 μm or less, and a proportion, in the fibrous copper microparticles, of copper particles having a minor axis of 0.3 μm or more and an aspect ratio of 1.5 or less is 0.1 or less per one fibrous copper microparticle.
 3. An electrically conductive coating agent comprising the coated fibrous copper microparticles according to claim
 1. 4. An electrically conductive layer comprising the coated fibrous copper microparticles according to claim
 1. 5. An electrically conductive film comprising a substrate, and the electrically conductive layer according to claim 4, formed on the substrate.
 6. An electrically conductive coating agent comprising the coated fibrous copper microparticles according to claim
 2. 7. An electrically conductive layer comprising the coated fibrous copper microparticles according to claim
 2. 8. An electrically conductive film comprising a substrate, and the electrically conductive layer according to claim 7, formed on the substrate. 